1. Field of the Invention
This invention relates to a process for producing a substrate for optical recording medium (optical disk recording medium) which has excellent resistance to abrasion or scratching, resistance to steam-permeation and adhesion to an information-recording layer, comprising forming a surface-hardened layer on the surface of a cast-cured product of an epoxy resin composition simultaneously with the casting of the epoxy resin composition.
1. Description of the Prior Art
The substrate which has heretofore been used in such an optical recording medium is a glass plate or a transparent synthetic resin plate, and there has been used a material which is optically isotropic in order to satisfy the requirement that the substrate be free from double refraction (birefringence). The transparent synthetic resin plate has been a molded article of polymethyl methacrylate (PMMA), polysulfone (PS), polycarbonate (PC), polyvinyl chloride (PVC), a copolymer of vinyl chloride and vinyl acetate or the like.
At present, a PMMA substrate formed by injection molding is in most cases used in view of moldability and optical transparency.
However, in the case of PMMA substrate, the enhancement of recording density is required, and the following disadvantages have become a great problem because high reliability of medium is required:
(a) It is inferior in thermal resistance, so that when the temperature of the surface of the substrate rises when vapor-depositing various functional films thereon, the guide groove becomes flat and unsatisfactory.
(b) It has a great moisture absorption, and therefore, deformation due to moisture absorption occurs and oxidation of the functional film becomes accelerated.
(c) Since the substrate is prepared by injection molding, stress due to the molding operation and double refraction appears.
(d) The substrate surface is inferior in resistance to abrasion or scratching, and when producing an optical recording medium from the substrate, or after the production process, the substrate tends to be flawed, which becomes a source of noise, and therefore, the resulting optical recording medium is inferior in reliability.
Moreover, the surface hardness of a substrate for optical recording medium consisting of a cast-cured product of an epoxy resin composition comprising an alicyclic epoxy resin or a mixture thereof with an aromatic epoxy resin, an organic polybasic acid anhydride, a curing accelerator and an anti-discoloring (anti-oxidative) agent is substantially the same as that of PMMA, and therefore, the surface of the substrate tends to be flawed. Therefore, said substrate as such is not satisfactory.
It has been considered that if at least the surface of a substrate for optical recording medium which is exposed to a laser light is subjected to a surface-hardening treatment, an optical recording medium excellent in handling could be obtained.
Moreover, it has also been considered that if one surface or both surfaces of a substrate for optical recording medium are subjected to surface-hardening treatment, the surfaces could be prevented from being flawed in the step of forming a functional film of tellurium or bismuth and the adhesion between the surface and the functional film could be enhanced.
According to a conventional method for the surface-hardening treatment, however, a surface-hardened layer of an organic compound has been applied directly to the surface of a substrate of PMMA or PC obtained by injection molding, by a spinner method, a dipping method or a roll-coater method.
In general, the conventional surface-hardening treatment has been conducted as follows: A substrate for optical recording medium is first prepared by casting, then taken out of the mold and thereafter subjected to a surface-hardening treatment. However, such a method has the disadvantage that the surface of the substrate exposed and therefore is subjected to the danger of becoming damaged.